Springtime HONO budget and its impact on the O₃ production in Zibo, Shandong, China

Nitrous acid (HONO) is a vital source of OH radicals, nevertheless, the potential sources of HONO are still uncertain. To get an insight into the HONO sources and its role in atmospheric photochemistry, a comprehensive field campaign was conducted in Zibo City of the North China Plain in April 2021. A high average HONO concentration of 0.91 ppb was observed during spring. Direct emission was an important HONO source in Zibo City, with an average contribution of 32%. A calculated NO2 heterogeneous conversion efficiency of 0.37% h(-1) indicated its limited contribution to the HONO formation. In addition, an unknown HONO source with an average production rate of 1.28 ppb h(-1) was found through the daytime budget analysis, and the correlation analysis suggested that the photo-enhanced NO2 heterogeneous reaction on the ground surface and the nitrate (NO3-) photolysis might be the daytime HONO missing source. After incorporating the contributions of the two sources, 65% of the HONO unknown source could be explained, while there was still an unknown source with a production rate of 0.45 ppb h(-1) that needed to be further explored. Furthermore, a 0-D box model was used to evaluate the role of HONO in O-3 formation chemisry. The results showed that the absence of HONO led to an 83% underestimation of the OH production rate and a 45% underestimation of the net O-x production rate. This study strengthened the understanding of the HONO sources and its impact on atmospheric photochemistry in Zibo City of the North China Plain.